Improved Jsc in CIGS thin film solar cells using a transparent conducting ZnO:B window layer

A comparative study of the cell performance of CIGS thin-film solar cells fabricated using ZnO:Al and ZnO:B window layers has been carried out. ZnO:B films were deposited by RF magnetron sputtering using an undoped ZnO target in a B₂H₆–Ar gas mixture. The short-circuit current (J_sc) was found to improve upon the replacement of the ZnO:Al layer with ZnO:B layers. This improvement in J_sc is attributed to an increase in quantum efficiency due to the higher optical transmission of the ZnO:B layer in the near-infrared region. The best cell fabricated with a MgF₂/ZnO:B/i-ZnO/CdS/CIGS/Mo structure yielded an active area efficiency of 18.0% with V_oc=0.645 V, J_sc=36.8 mA/cm², FF=0.76, and an active area of 0.2 cm² under AM 1.5 illumination.     

Glass fiber entrapped sorbent for reformates desulfurization for logistic PEM fuel cell power systems

Glass fiber entrapped ZnO/SiO₂ sorbent (GFES) was developed to remove sulfur species (mainly hydrogen sulfide, H₂S) from reformates for logistic PEM fuel cell power systems. Due to the use of microfibrous media and nanosized ZnO grains on highly porous SiO₂ support, GFES demonstrated excellent desulfurization performance and potential to miniaturize the desulfurization reactors. In the thin bed test, GFES (2.5 mm bed thickness) attained a breakthrough time of 540 min with up to 75% ZnO utilization at 1 ppm breakthrough. At equivalent ZnO loading, GFES yielded a breakthrough time twice as long as the ZnO/SiO₂ sorbent; at equivalent bed volume, GFES provided a three times longer breakthrough time (with 67% reduction in ZnO loading) than packed beds of 1–2 mm commercial extrudates. GFES is highly regenerable compared with the commercial extrudates, and can easily be regenerated in situ in air at 500 °C. During 50 regeneration/desulfurization cycles, GFES maintained its desulfurization performance and structural integrity. A composite bed consisting of a packed bed of large extrudates followed by a polishing layer of GFES demonstrated a great extension in gas life and overall bed utilization. This approach synergistically combines the high volume loading of packed beds with the overall contacting efficiency of small particulates.     

Nano-structured Cu2O solar cells fabricated on sparse ZnO nanorods

Nano-structured Cu₂O/ZnO nanorod (NR) heterojunction solar cells fabricated on indium tin oxide (ITO)-coated glass are studied. Substrate film and NR density have a strong influence on the preferred growth of the Cu₂O film. The X-ray diffractometer (XRD) analysis results show that highly (2 0 0)-preferred Cu₂O film was formed when plating on plain ITO substrate. However, a highly (1 1 1)-preferred Cu₂O film was obtained when plating on sparse ZnO NRs. SEM, TEM and XRD studies on sparse NR samples indicate that the Cu₂O nano-crystallites mostly initiate its nucleation on the peripheral surfaces of the ZnO NRs, and are also highly (1 1 1)-oriented. Solar cells with ZnO NRs yielded much higher efficiency than those without. In addition, ZnO NRs plated on a ZnO-coated ITO glass significantly improve the shunt resistance and open-circuit voltage (V_oc) of the devices, with consistently much higher efficiency obtained than when ZnO NRs are directly plated on ITO film. However, longer NRs do not improve the efficiency due to low short-circuit current (J_sc) and slightly higher series resistance. The best conversion efficiency of 0.56% was obtained from a Cu₂O/ZnO NRs heterojunction solar cell fabricated on a 80 nm ZnO-coated ITO glass with V_oc=0.514 V, J_sc=2.64 mA/cm² and 41.5% fill factor.     

Investigation of WO3/ZnO thin-film heterojunction-based Schottky diodes for H2 gas sensing

A comparative study of Schottky diode hydrogen gas sensors based on Pd/WO₃/Si and Pd/WO₃/ZnO/Si structure is presented in this work. Atomic force microscopy and X-ray photoelectron spectroscopy reveal that the WO₃ sensing layer grown on ZnO has a rougher surface and better stoichiometric composition than the one grown on the Si substrate. Analysis of the I–V characteristics and dynamic response of the two sensors when exposed to different hydrogen concentrations and various temperatures indicate that with the addition of the ZnO layer, the diode can exhibit a larger voltage shift of 4.0 V, 10 times higher sensitivity, and shorter response and recovery times (105 s and 25 s, respectively) towards 10,000-ppm H₂/air at 423 K. Study on the energy band diagram of the diode suggests that the barrier height is modulated by the WO₃/ZnO heterojunction, which could be verified by the symmetrical sensing properties of the Pd/WO₃/ZnO/Si gas sensor with respect to applied voltage.     

Cu(In,Ga)Se2 solar cells on stainless steel substrates covered with insulating layers

We have developed the flexible Cu(In,Ga)Se₂ (CIGS) solar cells on the stainless steel substrates with the insulating layer for the fabrication of the integrated module. The CIGS films have strong adhesion to the Mo films with insulating layers. An efficiency of 12.3% was achieved by the flexible CIGS solar cell with a structure of ITO/ZnO/CdS/CIGS/Mo/SiO₂/stainless steel. The insertion of the SiO₂ insulating layer did not have an influence on the formation of the CIGS film and solar cell performances.     

Fabrication of wide-gap Cu(In1−xGax)Se2 thin film solar cells: a study on the correlation of cell performance with highly resistive i-ZnO layer thickness

The correlation of the cell performance of wide-gap Cu(In_1−xGa_x)Se₂ (CIGS) solar cells with the thickness of highly resistive i-ZnO layers, which are commonly introduced between the buffer layer and the transparent conductive oxide (TCO) layer in CIGS solar cell devices, was studied. It was found that cell parameters, in particular, the fill factor (F.F.) varied with the thickness of the i-ZnO layers and the variation of the F.F. was directly related to cell efficiency. A 16%-efficiency was achieved without use of an anti-reflection coating from wide-gap (E_g1.3 eV) CIGS solar cells by adjusting the deposition conditions of the i-ZnO layers.     

A photovoltaic cell incorporating a dye-sensitized ZnS/ZnO composite thin film and a hole-injecting PEDOT layer

A photovoltaic cell containing a dye-sensitized ZnS/ZnO composite thin film was studied. ZnS was thermally evaporated or electrodeposited onto conducting fluorine-doped tin oxide glass; then a particulate ZnO layer was pasted and sintered to form a ZnS/ZnO composite layer. A visible light source was utilized to excite the Ru-dye, which was adsorbed onto the surface of the ZnO. The ZnS layer is believed to provide an alternative pathway for electrons to move across ZnO barriers. This alternative pathway with the composite layer structure provides higher power efficiency than does a single layer of ZnO or ZnS. A hole-injecting, p-type poly(3,4-ethylenedioxythiophene) (PEDOT) thin film was also introduced to substitute for the Pt catalytic layer which helps with the rejuvenation of I⁻ ions. Although the p-type semiconductor behavior increased the open circuit voltage (V_oc), the power efficiency decreased because the I⁻ rejuvenation rate was much slower on PEDOT than on Pt.     

Growth of highly c-axis oriented Mg:ZnO nanorods on Al2O3 substrate towards high-performance H2 sensing

Highly c-axis oriented Mg:ZnO films were fabricated on Al₂O₃ substrate by radio frequency sputtering for different substrate temperatures. The crystal structure revealed that the Mg dopants are well integrated into ZnO wurtzite lattice. X-ray photoelectron spectroscopy measurements also confirmed the successful incorporation of Mg into ZnO. The substrate temperature exhibit significant influence on the optical absorbance and band gap of Mg:ZnO films. Scanning electron microscope images revealed the formation of Mg:ZnO nanorods with good crystalline quality. The films prepared at 1200 °C show well grown rods of Mg:ZnO due to strengthening of the preferred orientation of ZnO along the c-axis. The Mg:ZnO/Al₂O₃ films prepared at different temperature were tested for its sensing performance towards 200 ppm of H₂ at room temperature. The Mg:ZnO sensor prepared at 1200 °C revealed fast response and recovery time of about 85 s and 70 s, respectively. The response of the sensor was linear to H₂ concentration in the range of 100–500 ppm. It can be summarized that this high performance H₂ sensor has potential for use as a portable room temperature gas sensor.     

Structure and optical properties of ZnO nanowires coated with silica

Abstract

Abstract

It is essential to passivate one-dimensional nanostructures with insulating materials to protect them from contamination and oxidation as well as to avoid cross-talking between the building blocks of complex nanoscale circuits. The ZnO nanowires synthesised by the thermal evaporation of ZnO powders were coated with SiO₂ by the sputtering technique. Transmission electron microscopy and X-ray diffraction analyses revealed that the cores and shells of the ZnO core–SiO₂ shell nanowires were single crystal wurtzite type ZnO and amorphous SiO₂ respectively. Photoluminescence measurements at room temperature showed that the passivation of the ZnO nanowires was successfully achieved with SiO₂ without nearly degrading the near band edge emission from the wires. However, subsequent thermal annealing treatment was found to be undesirable owing to the degradation of the near band edge emission in intensity.     

ZnO/CuInS2 core/shell heterojunction nanoarray for photoelectrochemical water splitting

ZnO/CuInS₂ core/shell nanorods array thin film was synthesized on conducting glass substrates for photoelectrochemical water splitting via a simple hydrothermal and cation exchange reaction, using ZnO nanorods array as reactive template. Uniform CuInS₂ films were obtained on the surface of ZnO nanorods, based on the ion-by-ion growth mechanism. The optical property of core/shell nanoarray was characterized, and enhanced absorption spectrum was observed. Hydrogen generation efficiency of 3.2% at 0.29 V versus saturated calomel electrode was achieved with synthesized ZnO/CuInS₂ core/shell nanoarray electrode due to the improved absorption and appropriate energy gap structure. The synthesized core/shell nanoarray has potential application in photoelectrochemical water splitting.     

High-efficiency inverted polymer solar cells with solution-processed metal oxides

The selection of carrier transporting layer in polymer solar cells is an important issue because the nature and direction of carrier transport can be manipulated by inserting different functional layers in the device structure. In this work, we report a very efficient inverted polymer solar cell (PSC) system based on regioregular poly(3-hexylthiophene) and a n-type acceptor, bis-indene[C₆₀]. With a pair of metal oxides and the insertion of TiO₂ nanorods electron collecting layer between the ZnO thin film and the active layer, the device efficiency can be greatly improved. The contact area between the active layer and the electron collecting layer, as well as the thickness of active layer, can be increased with the incorporation of TiO₂ nanorods. As a result, photocurrent can be enhanced due to more absorption of light and more charge separation interface. In addition, the larger contact area and the crystalline TiO₂ nanorods provide a more efficient transporting route for the carriers to the cathode. The most efficient device demonstrated shows a high power conversion efficiency of 5.6% with the inverted structure.     

Influence of In2S3 film properties on the behavior of CuInS2/In2S3/ZnO type solar cells

Solar cells of CuInS₂/In₂S₃/ZnO type are studied as a function of the In₂S₃ buffer deposition conditions. In₂S₃ is deposited from an aqueous solution containing thioacetamide (TA), as sulfur precursor and In³⁺. In parallel, variable amounts of In₂O₃ are deposited that have an important influence on the buffer layer behavior. Starting from deposition conditions determined in a preliminary study, a set of parameters is chosen to be most determining for the buffer layer behavior, namely the solution temperature, the concentration of thioacetamide [TA], and the buffer thickness. The solar cell results are discussed in relation with these parameters. Higher efficiency is attained with buffer deposited at high temperature (70 °C) and [TA] (0.3 M). These conditions are characterized by short induction time, high deposition rate and low In₂O₃ content in the buffer. On the other hand, the film deposited at lower temperature has higher In₂O₃ content, and gives solar cell efficiency sharply decreasing with buffer thickness. This buffer type may attain higher conversion efficiencies if deposited on full covering very thin film.     

High efficiency cadmium free Cu(In,Ga)Se2 thin film solar cells terminated by an electrodeposited front contact

The possibility to reach up to 14.7% efficiency with Cu(In,Ga)Se₂ (CIGS) solar cell, using a cadmium free buffer layer (indium sulphide:In₂S₃) and an electrodeposited front contact (chloride doped ZnO:ZnO:Cl) is demonstrated in this article. This is the first time that costly gas phase deposition processes for ZnO, by high vacuum sputtering, can be replaced by an efficient low cost atmospheric technology, representing an important breakthrough in further cost reduction for photovoltaic application. In addition, the compatibility with cadmium free buffer layers brings this new approach at the cutting edge of strategic evolution of the CIGS technology. In this study the influences of the In₂S₃ buffer layer thickness, the presence of an intrinsic ZnO layer and a soft annealing treatment are studied. It is shown that the growth behavior of the electrodeposited ZnO:Cl is controlled by nucleation phenomena on different surfaces, with a unique morphology on indium sulphide. Finally the best performances have been achieved with a cell annealed at 150 °C under atmospheric conditions containing a very thin In₂S₃ layer (15 nm) but without intrinsic ZnO (CIGS/In₂S₃/ZnO:Cl).     

Preparation of ZnO thin films using MOCVD technique with D2O/H2O gas mixture for use as TCO in silicon-based thin film solar cells

Zinc oxide (ZnO) thin films have been successfully grown by metal organic chemical vapor deposition (MOCVD) technique using deuterium water (D₂O) and water (H₂O) mixtures as oxidants for diethylzinc (DEZ). B₂H₆ was also employed as a dopant gas. It was found that the crystal orientation of ZnO films strongly depends on D₂O/H₂O ratio. As a result, the surface morphology of ZnO changed from textured surface morphology to smooth surface morphology with increase in the ratio of D₂O/H₂O. Moreover, it was also observed that the carrier concentration of ZnO films did not change with the ratio of D₂O/H₂O, while the mobility of these films was strongly dependent on the D₂O/H₂O ratio. Without D₂O addition, the resistivity of films had its lowest value and the minimum sheet resistance was 10 Ω/square. All films showed transmittance higher than 80% in the visible region. Moreover, the haze values of these films could be controlled by the ratio of D₂O/H₂O. These results indicate that the crystal orientation and surface morphology of the low resistivity ZnO films can be modified by using a mixture of D₂O and H₂O without changing the deposition temperature. Thus, the obtained ZnO films are promising for use as a front TCO layer in Si-based thin film solar cells.     

TiO2/ZnO/TiO2 sandwich multi‐layer films as a hole‐blocking layer for efficient perovskite solar cells

A continuous and compact hole‐blocking layer is crucial to prevent photocurrent recombination at the photoanode/electrode interface of high‐performance mesostructure perovskite‐based solar cells. Novel TiO₂/ZnO/TiO₂ sandwich multi‐layer compact film prepared as hole‐blocking layer for perovskite solar cell. Herein, TiO₂, ZnO, and TiO₂ layers were successfully deposited by spin‐coating onto FTO glass substrate in sequence. The fill factor and power conversion efficiency of the perovskite solar cell are remarkably improved by the employment of a TiO₂/ZnO/TiO₂ sandwich compact layer. Perovskite solar cell based on TiO₂/ZnO/TiO₂ sandwich film has been observed to exhibit maximum incident‐photon‐to‐current conversion efficiency in the visible region (400–780 nm) and reach a power conversion efficiency of 12.8% under AM1.5G illumination. Copyright © 2016 John Wiley & Sons, Ltd.     

Fabrication of ZnO/SrTiO3 nanoarrays and its photoelectrochemical performances

The ZnO/SrTiO₃ nanomaterials were fabricated by a chemical conversion hydrothermal method in order to utilize the high electron transfer rate of one-dimensional ZnO nanorods and photocatalytic activity of SrTiO₃. The technological parameters, such as TiO₂ sol concentration, TiO₂ sol dipping cycle, Sr(NO₃)₂ concentration and reaction temperature, were investigated in the synthetic process and the reaction mechanism of the ZnO/SrTiO₃ nanomaterials was proposed. A photocurrent density of 7.53 mA/cm² was obtained for the as-prepared ZnO/SrTiO₃ photocatalyst, attributed to its improved absorption spectrum and appropriate nanostructure, which indicates a potential application in photoelectrochemical water splitting.     

Improved compositional flexibility of Cu(In,Ga)Se2-based thin film solar cells by sodium control technique

The effects of sodium on off-stoichiometric Cu(In,Ga)Se₂ (CIGS)-based thin films and solar cells were investigated. The CIGS-based films were deposited with intentionally incorporated Na₂Se on Mo-coated SiO_x/soda-lime glass substrates by a multi-step process. By sodium control technique high-efficiency ZnO : Al/CdS/CIGS solar cells with efficiencies of 10–13.5% range were obtained over an extremely wide Cu/(In + Ga) ratio range of 0.51–0.96, which has great merit for the large-area manufacturing process. The improved efficiency in the off-stoichiometric regions is mainly attributed to the increased acceptor concentration and the formation of the Cu(In,Ga)₃Se₅ phase films with p-type conductvity. A new type of solar cell with p-type Cu(In,Ga)₃Se₅ phase absorber materials is also suggested.     

Catalytic properties of Ag promoted ZnO/Al2O3 catalysts for hydrogen production by steam reforming of ethanol

Ag promoted ZnO/Al₂O₃ catalysts were prepared by using the incipient wetness impregnation method. The catalytic properties of steam reforming reaction for hydrogen production on the prepared catalysts were evaluated with H₂O:C₂H₅OH molar ratios of 3:1 at 450 °C and atmospheric pressure. Ag promoted ZnO/Al₂O₃ catalysts show higher SRE catalytic activity than ZnO/Al₂O₃ catalysts. H₂ and CH₃CHO are the major products on Ag promoted catalysts, and C₂H₄ is also produced probably due to acid sites on Al₂O₃. SRE mechanism on Ag promoted ZnO/Al₂O₃ catalysts, which contains C-C scission, is different from that on ZnO/Al₂O₃ catalysts. A method based on thermogravimetry (TG), differential scanning calorimetry (DSC) and mass spectrometry (MS) was used to analysis the coking behavior on catalyst surface. The surfaces of Ag promoted ZnO/Al₂O₃ catalysts show two different types of coking, and suffer higher coke deposition during the steam reforming reaction.     

Catalytic conversion of CH4 and CO2 to synthesis gas on Ni/SiO2 catalysts containing Gd2O3 promoter

A series of Ni/SiO₂ catalysts containing different amounts of Gd₂O₃ promoter was prepared, characterized by H₂-adsorption and XRD, and used for carbon dioxide reforming of methane (CRM) and methane autothermal reforming with CO₂ + O₂ (MATR) in a fluidized-bed reactor. The results of pulse surface reactions showed that Ni/SiO₂ catalysts containing Gd₂O₃ promoter could increase the activity for CH₄ decomposition, and Raman analysis confirmed that reactive carbon species mainly formed on the Ni/SiO₂ catalysts containing Gd₂O₃ promoter. In this work, it was found that methane activation and reforming reactions proceeded according to different mechanisms after Gd₂O₃ addition due to the formation of carbonate species. In addition, Ni/SiO₂ catalysts containing Gd₂O₃ promoter demonstrated higher activity and stability in both CRM and MATR reactions in a fluidized bed reactor than Ni/SiO₂ catalysts without Gd₂O₃ even at a higher space velocity.     

Properties of ZnO thin films prepared by reactive evaporation

Non-doped ZnO thin films with suitable characteristics to be used as transport contact and as buffer layer in solar cells, were prepared by reactive evaporation. Through a parameter study, it was found that the main deposition parameters affect the optoelectrical properties of the ZnO films, being the oxygen partial pressure the parameter which most affects both, the transmittance and the conductivity. Actually, high-conductive ZnO films with blue transmittances greater than 80% are routinely prepared by using oxygen partial pressures greater than 0.2 mbar and evaporation temperatures of Zn about 540°C. AFM measurements revealed that the high values of blue transmittance obtained with ZnO film deposited at high O₂ – partial pressure are in part controlled by morphological effects. From Hall coefficient and conductivity measurements it was found that the conductivity is dominated by the resulting carrier concentration which is controlled by oxygen vacancies. The conditions to prepare in situ the double low ρ-ZnO/high ρ-ZnO bilayer structure regularly used in the fabrication of CdS-free, thin films solar cells, were also found through this study.